A fuel cell has characteristics that it is high in efficiency because it can take out electric energy directly from free energy changes caused by combustion of fuel. Further, the fuel cell does not discharge any harmful substance and thus have been extended to be used for various purposes. In particular, a solid polymer electrolyte fuel cell has characteristics that it is high in power density and compact in size and operates at low temperatures.
A fuel gas for a fuel cell generally contains hydrogen as the main component. Examples of raw materials of the fuel gas include hydrocarbons such as natural gas, LPG, naphtha, and kerosene; alcohols such as methanol and ethanol; and ethers such as dimethyl ether. However, elements other than hydrogen are present in the aforesaid raw materials and thus impurities of carbon origin can not be avoided from mixing in the fuel gas to be supplied to a fuel cell.
Carbon monoxide in particular poisons a platinum-based metal used as an electrocatalyst of a fuel cell. Therefore, if carbon monoxide is present in a fuel gas, the fuel cell would not be able to obtain sufficient power-generating characteristics. In particular, a fuel cell operating at lower temperatures undergoes carbon monoxide absorption and thus is more likely to be poisoned. It is, therefore, indispensable to decrease the concentration of carbon monoxide in the fuel gas for a system using a solid polymer electrolyte fuel cell.
It is contemplated that a method, so-called “water-gas-shift reaction” wherein carbon monoxide in a reformed gas produced by reforming a raw material reacted with steam to convert them to hydrogen and carbon dioxide be used in order to reduce the concentration of carbon monoxide. However, this method can reduce the carbon monoxide concentration only down to 0.5 to 1 percent by volume. Therefore the carbon monoxide concentration having been reduced to 0.5 to 1 percent by volume by the water-gas-shift reaction is required to be further reduced.
It is contemplated to use adsorption separation and membrane separation methods in order to further reduce the carbon monoxide concentration. However, these methods can provide high purity hydrogen but have problems that they are not suitable for actual use because the apparatuses for these methods are high in cost and large in size.
Whereas, it can be said that a method chemically reducing the carbon monoxide concentration does not encounter the above problems and thus is more realistic. Examples of such chemical methods include methanation of carbon monoxide and conversion of carbon monoxide to carbon dioxide by oxidation. Alternatively, a two-step method has been proposed, in which carbon monoxide is methanated at the first step and then oxidized at the second step (see Japanese Patent Application Laid-Open Publication No. 11-86892).